1. Field of the Invention
The present invention relates to a novel photopolymerization initiator that can be favorably used for photoresist materials, printing board materials, hologram materials and dental materials, and to a photopolymerizable composition containing the above photopolymerization initiator.
2. Description of the Related Art
There have heretofore been proposed a variety of photocationic polymerization initiators that generate Bronsted acid or Lewis acid, and that polymerize cationically polymerizable compounds such as epoxide or vinyl ether.
As such photocationic polymerization initiators, there can be exemplified photo acid-generating compounds such as iodonium salt compound and sulfonium salt compound. However, these photo acid-generating compounds do not usually absorb light in the near-ultraviolet region and cannot, hence, excite the polymerization reaction even by using an ordinary source of light (370 to 550 nm) such as halogen lamp. It has therefore been attempted to use a fused polycyclic aromatic compound having a substituent, such as alkoxy group, aralkyloxy group or acyloxy group as a sensitizing agent in combination with the photo acid-generating compound, so that it works as a photocationic polymerization initiator (see, for example, Non-Patent Documents 1 to 3, and Patent Documents 1 to 5). There has further been proposed the use of these fused polycyclic aromatic compounds in combination with a thioxanthone derivative.
There have also been known photo acid-generating compounds which, by themselves, absorb light in the near-ultraviolet to visible light regions, such as cyclopentadienyl/iron/allene complex, and diazonium salt compound (see, for example, Non-Patent Document 1).
Non-Patent Document 1;
                Ao Yamaoka, Gentaro Matsunaga, “Photopolymer Technology”, Nikkan Kogyo Shimbunsha, pp. 38-46Non-Patent Document 2;        The Society of Polymer Science, Japan, “Synthesis/Reaction of polymer (1), Synthesis of addition polymerized polymer”, Kyoritsu Shuppan Co., pp. 400-404Non-Patent Document 3;        Kazuhiko Morio, Hiroshi Tsuchiya, Tsuyoshi Endo, “Modern Progress of Photo-Initiated Cationic Polymerization”, Functional Material, CMC Co., Oct., 1985, pp. 5-13Patent Document 1;        Japanese Unexamined Patent Publications (Kokai) No. 11-199681Patent Document 2;        Japanese Unexamined Patent Publications (Kokai) No. 11-322952Patent Document 3;        Japanese Unexamined Patent Publications (Kokai) No. 2000-7716Patent Document 4;        Japanese Unexamined Patent Publications (Kokai) No. 2001-81290Patent Document 5;        Japanese Unexamined Patent Publications (Kokai) No. 11-263804        
In the field of dental materials, on the other hand, there has been widely employed a radically polymerizable system using a (meth)acrylate-type polymerizable monomer as applied to a variety of products. For the dental applications, it is a widely accepted practice to conduct the polymerization by using visible rays by taking the beneficial/harmful action to the living body into consideration, and there have been practically used a polymerization initiator comprising an combination of α-dicarbonyl compound and an aromatic amine, as well as a polymerization initiator comprising an acylphosphine oxide.
However, the radically polymerizing system has a problem in that the polymerization is impaired by oxygen and, besides, volumetric-shrinkage takes place to a large extent due to polymerization. A paste-type dental restorative as represented by a composite resin or a compmer(Polyacid modified resin composite) used for restoring the decay or breakage of natural teeth, is directly filled in a cavity of tooth, shaped and is, then, cured. Therefore, an unpolymerized layer or a lowly polymerized layer is produced in the surface of the cured product as the polymerization is impaired by oxygen. Accordingly, the surface must be polished after curing. When the polishing is not effected, there occurs such a problem in that the cured product is colored or discolored due to aging. Further, since the volumetric shrinkage undergoes to a large extent due to the polymerization, there develops a gap in the interface relative to the dentin, causing the falling of the restorative or the secondary decaying due to the infiltration of bacteria to the gap.
The cationic polymerization is superior to the radical polymerization from such a standpoint that the polymerization is not impaired by oxygen. In particular, when a compound that undergoes the ring-opening-polymerization is used as a polymerizable monomer, the problem of volumetric shrinkage due to the polymerization greatly decreases.
In the above-mentioned system comprising the photo acid-generating compound and the sensitizer, however, the rate of polymerization is low and the depth of curing is not sufficient. Therefore, this system does not fully work as a dental polymerization initiator for effecting the curing to a sufficient degree within a short period of time for the therapy in the oral cavity.
A system that absorbs light in the near-ultraviolet to visible ray region as a photo acid-generating compound, is not still usable for the dental applications. For example, a cyclopentadienyl/iron/allene complex is strongly colored after curing while a diazonium salt compound form bubbles.
In recent years, there have been proposed several cationic polymerization initiators for dental use improving the above-mentioned defects. For example, by using an α-dicarbonyl compound that is a known radical polymerization initiator in combination with an iodonium salt compound, a mixture of the epoxide and the hydroxy compound can be cured by the irradiation with visible rays to provide improved depth of curing (see, for example, Patent Document 6). Upon further combining the α-dicarbonyl compound/iodonium salt compound with aromatic amines, further, it has been reported that the curing rate can be further improved (see, for example, Patent Documents 7 and 8).
However, these cationic polymerization initiators don't exhibit a sufficient activity of polymerization and are practically used.
Patent Document 6;
                International Patent Publication No. 10-508067Patent Document 7;        Japanese Unexamined Patent Publication (Kokai) No. 11-130945Patent Document 8;        International Patent Publication No. 2002-500172        